- Pulmonary Hypertension refers to pulmonary arterial pressures above their normal value of 15 mm Hg. Pulmonary hypertension may be due to an idiopathic primary disease of the pulmonary vasculature or arise secondary to a variety of identifiable disease processes.
|Primary Pulmonary Hypertension|
- Primary Pulmonary Hypertension is a disease of unknown etiology and pathogenesis that typically affects women between 20 - 40 years of age. The disease may be caused by hyper-reactivity of the pulmonary vasculature, resulting in chronic pulmonary vasoconstriction and thus excessive pulmonary vascular resistance.
|Secondary Pulmonary Hypertension|
- A wide variety of etiologies can secondarily result in pulmonary hypertension and can be categorized according to their basic initial pathophysiological insult.
- Directly Increased Pulmonary Vascular Resistance
- Pulmonary vascular resistance may be directly increased in a number of scenarios. For example, chronic habitation at high altitude can result in long-term engagement of the hypoxic vasoconstriction response (See: Pulmonary Blood Flow Regulation), resulting in elevations of pulmonary vascular resistance. Repeated showers of small pulmonary emboli can progressively occlude increasing numbers of small pulmonary vessels, over time increasing pulmonary vascular resistance. Widespread destruction of small pulmonary vessels can occur in emphysema and a number of interstitial lung diseases to the point of increasing the organ's total vascular resistance.
- Increased Left Atrial Pressure
- Pathological increases in left atrial pressure occur due to a number of heart diseases including left heart failure, mitral stenosis, or chronic mitral regurgitation. These pressures are retrogradely transmitted to the pulmonary veins; consequently, the right heart together with the pulmonary circulation boost the pulmonary arterial pressure in order to maintain a sufficient pressure gradient to actuate blood flow through the pulmonary circulation. Although this increased pressure gradient is able to maintain cardiac output, it necessitates the maintenance of chronically increased pulmonary arterial pressures, or in other words pulmonary hypertension.
- Increased Pulmonary Blood Flow
- A variety of diseases that cause left-right shunts, result in pulmonary hypertension if the shunt is sustained over time. Recall that left-right shunts result in blood volume from the left heart recirculating through the pulmonary vasculature, necessitating that the pulmonary circulation transmit the total cardiac output in addition to the the shunted volume. Over time, this supra-normal blood flow through the pulmonary circulation results in widespread remodeling of the pulmonary arterioles such that pulmonary vascular resistance increases, thus ultimately yielding pulmonary hypertension.
- Regardless of the cause of pulmonary hypertension, some common morphological changes occur simply due to the chronically increased pulmonary arterial pressure. These changes are found throughout the pulmonary arterial tree include hypertrophy of the tunica intima and tunica media to the point of occlusion or near-occlusion of the vascular lumen. In long standing cases, some arterioles may display "Plexiform" lesions which may represent previously thrombosed and re-canalized vessels.